526 MR MALCOLM LAURIE ON THE 



Other Arachnids. 



It is usually taken for granted that once the ancestry of the Scorpion is settled, the 

 ancestry of all the terrestrial Arachnids is fixed, but there seem to me to be good grounds 

 for dissenting from this point of view.* As I hope shortly to publish a paper with full 

 anatomical details of the recent forms, I will only give here a brief resume of some of the 

 points which seem to me to show a relationship with the Eurypterids. 



In Thelyphonus (PI. II. fig. 15) and Phrynus we have arachnids as primitive in most 

 respects as Scorpio. The body consists of a carapace bearing six pairs of limbs on its 

 under side, and followed by twelve free segments. One chief difference, however, between 

 this form and Scorpio is that Thelyphonus has, when examined on the ventral surface, 

 apparently only five abdominal segments, the ventral portion corresponding to the first two 

 tergites being covered by a single plate, beneath which is the aperture of the generative 

 organs. Thelyphonus further differs from Scorpio in having only two pair of lung-books, 

 the more anterior of which lie beneath the large genital plate, while the second pair lie 

 beneath the second ventral sclcrite, i.e., that belonging to the third segment. Now, this 

 suppression of the sclerite of the second abdominal segment — its ventral surface being 

 covered by the genital plate — is exactly what we find in Eurypterids, and very different 

 from the condition of things in Scorpio. Further, if the two pairs of lung-books of Thely- 

 phonus correspond to the anterior two pairs of Scorpio, then the first pair is shifted from 

 its proper position on the third segment, and lies right forward in the second. My 

 interpretation of this — based upon as complete a study of the anatomy and development of 

 these forms as the material at my command would permit of — is that in the first pair of 

 lung-books of Thelyphonus we have the homologues of the pectines of Scorpions, and of 

 the branchial lamellae, found beneath the genital operculum in Slimonia, &c, while the 

 second pair of lung-books correspond to the first pair of Scorpio, the second sclerite being, 

 like the first, an appendage, and not part of the body-wall. If this view be correct it 

 would mean that the Pedipalpi arose after the great development of the genital plate 

 which is characteristic of Eurypterids. The chief difficulties which this view involves 

 seem to me to be (1) the resemblance between the lateral eyes of Scorpio and Thely- 

 phonus, and (2) the fact that it requires lung-books to have been developed from gills 

 twice over. These difficulties I hope to meet in a future paper. It is unfortunate that our 

 knowledge of the Anthracomarti is too fragmentary to enable any deductions to be safely 

 drawn as to their position. 



If the above views are correct, it would tend to separate Glyptoscorpius from the 

 Eurypteridae along with the Scorpions. I do not see any difficulties in the way of this, 



* Since the above was written, Mr R. I. Pocock has published a paper on the "Morphology of the Arachnidi'' 

 (.f »//. and Mag.) vol. xi.), in which he advocates the division of the Arachnida into two sub-classes, one which he terms 

 Ctenophora containing the Scorpiones, and the other — the Lipoctena — containing the rest of the class. This division 

 entirely agrees with my views, but it is unfortunate that he should have chosen Ctenophora as the name of a sub- 

 class, as it is already accepted as the" name for a class of the Coolenterata. 



